The invention relates to an applicator system for application of color coating on a paper web. The system may include both roll type and nozzle type applicators.
Roll applicators of this type are known from DE 36 05 409 A1. This device is intended to produce a uniform coating.
The technical requirements for coating systems, more generally, are always the following: the coating is to be applied on the paper web uniformly in any respect, and at that, both in cross direction (cross profile) and also in longitudinal direction. The primary concern is the coating weight, but also other properties such as the outer appearance of the coating surface.
The disturbance factors opposing these requirements are numerous. With roll type applicator systems, problems occur most of all in the wedge-shaped entrance zone between applicator roll and backing roll, and at that, especially at high velocities. This is attributable most of all to the effect of the air which in the rotation of the backing roll is carried into the entrance bore together with the paper web. The air mixes with the color coating at the point where the latter makes contact with the paper web. Generally, an overflow of color coating occurs at the upper edge of the machinewide guide assembly, and at that, opposite to the direction of travel of the paper web and opposite to the airflow. If the airflow impinges on this overflow, the result is a partial backup of the overflow as well as a mixing of air and color coating. Disturbances of that type occur also in the absence of a guide assembly, when as a consequence an unimpeded level of coating mixture adjusts itself between the shell surface of the applicator roll and the wall of the trough.
Another disturbance factor may be constituted by an irregular supply of color coating to the application zone. These irregularities may have various causes, for instance manufacturing inaccuracies of the participating components, shortcomings in the design geometry or flow geometry, or hydrodynamic disturbance problems, and thus undesirable variations of the dimensions of the flow channels in which the color coating flows.
A third category of disturbance factors resides in the base paper. As is generally known, the base paper involves fluctuations of the basis weight, roughness and absorption performance, and at that, both across the web width and also in the travel direction of the web, and thus over time. These factors are particularly difficult to manage.
As known, doctor systems are located at the end of the usual roll type applicators. These systems comprise a doctor blade, which can be set at the paper web bearing on the backing roll, and a doctor beam supporting the blade. In the event of coating irregularities, corrections are attempted by means of the doctor system. In doing so, an adjustment can be effected across the entire width. But locally limited corrections can also be made, that is, only at certain points of the web width. These measures may provide a certain remedy, but mostly they are insufficient to achieve the desired result.
The problem underlying the invention is to design an applicator system in such a way that the coating may in any respect be produced more uniformly than was the case previously.